The brushless motor generally includes a position sensor for detecting a rotational position of a rotor, of which rotational position and rpm can be controlled by driving the windings of a stator with an energizing timing determined by positional information detected by the position sensor. However, a misalignment of the position sensor from a given position will invite a failure in achieving accurate rotational control over the brushless motor, thereby lowering a rotational efficiency or increasing a rotational sound.
To overcome these problems, a technique has been proposed such as positional information supplied from the position sensor is corrected to an appropriate energizing timing before the windings are driven (e.g. refer to Patent Literature 1.) This conventional technique is embodied by providing the motor control circuit with a motor current detecting means and a correcting means for correcting an energizing timing. The motor control circuit performs a control such that a motor current value detected by the motor current detecting means should be minimized as well as the correcting means should correct the energizing timing. To be more specific, the motor control circuit searches optimum phase correction amounts in every possible operating state, and then stores these amounts in a memory. The motor is driven by using the phase correction amounts stored in the memory, and then the correction amounts are fluctuated appropriately for finding an optimum phase correction value that allows minimizing the motor current value. In other words, a conventional motor control circuit drives a motor with a lead-angle amount (a lead-angle advancing a phase of motor current) that reduces a motor current.
The structure of the foregoing conventional instance allows minimizing an adverse effect caused by dispersions in a circuit constant, a motor winding constant, a DC voltage, and a motor current, whereby the rotational control of the motor can be done steadily.
However, as the forgoing conventional instance tells, although a minimum value of motor current is tried to detect, it cannot be accurately detected. FIG. 9A shows a theoretical relation of characteristics between a lead-angle amount corresponding to an energizing timing and a motor current. As shown in FIG. 9A, the characteristics bows downward while the lead-angle amount is changed with an rpm kept constant, and a minimum value of the motor current is found. In other words, driving the windings with the lead-angle amount at which the motor current value is minimized allows the motor to be driven efficiently. FIG. 9B shows instances of measuring the motor current value while the lead-angle amount is changed with an rpm kept constant. As shown in FIG. 9B, the measurement of the motor current value proves it is difficult to obtain the theoretical characteristics of the downward bow. In other words, the motor current value close to the minimum value is susceptible to noise comparing with a large current, and a change in current value relative to a change in the energizing timing is small. As a result, the minimum value of the motor current is difficult to detect during the measurement of the motor current. The conventional structure thus sometimes fails in achieving an optimum phase correction amount or sometimes produces sound noises due to frequent changes in the phase correction amount.    Patent Literature 1: Unexamined Japanese Patent Application No. H06-284782